1. Field of the Invention
The present invention relates to an image heating apparatus for heating an image on a recording material. This image heating apparatus is used in a copying machine, a printer, fax machine and so forth, which form an image, for example, by an electrophotographic method.
2. Description of the Related Art
In an electrophotographic image forming device, heat and pressure is applied, normally by a fixing apparatus, to a sheet bearing a transferred toner image to fix the toner image permanently to the sheet.
Specifically, a fixing roller including a heater in the inside and a pressurizing roller are arranged to contact each other under pressure so as to form a nip part, and the sheet is passed through the nip part to fix the toner image.
Further, for purposes of improving an energy consumption efficiency and enhancing a fixing characteristic, a fixing apparatus uses thin cylindrical films in place of the fixing roller and the pressurizing roller. Besides, instead of using the heater, another fixing apparatus performs an induction heating of a metallic member.
In any of such fixing apparatuses, when a sheet that is narrower than a heating region is used, heat is not absorbed from a non-sheet-passing region by the sheet while heat is absorbed from a sheet-passing region by the sheet. Consequently, the temperature at the non-sheet-passing region rises excessively.
This excessive temperature rise may accelerate deterioration of the rollers or the films. When a planar heater including a. ceramic material and the like is used as a base material, the excessive temperature rise may cause cracking in the heater. Besides, when a wide sheet is fixed after the temperature rise occurs, a drawback of high temperature offset occurs. In the high temperature offset, toners on the sheet attach to the fixing roller or the fixing film, and stain an image on the sheet where (mainly the non-sheet-passing region) the excessive temperature rise occurs.
In order to overcome these drawbacks, according to Japanese Patent Application Laid-Open No. 6-149103 (see corresponding U.S. Pat. No. 5,669,039A), when images are formed on narrow sheets continuously, the sheets are fed at extended intervals and power supply to a heater is cut off between the sheet feedings. In this manner, temperature falls at a non-sheet-passing part of a fixing apparatus.
Besides, in Japanese Patent Application Laid-Open No. 1-121883, a cooling roller through which fluid flows contacts a pressurizing roller to remove heat of the pressurizing roller so as to prevent temperature from rising excessively in a non-sheet-passing region at an end of a fixing apparatus.
However, the above-related art involve drawbacks as follows. In order to prevent excessive temperature rise in the non-sheet-passing region, the feeding intervals are extended. In that case, throughput (number of sheets output per unit time) may have to be reduced to ½ or less of normal throughput, depending on widths and numbers of the sheets. Therefore, productivity of an image forming device may decrease.
In the structure in which the cooling roller cools the pressurizing roller, the rotatable cooling roller itself contacts to the pressurizing roller. Therefore, it is required to prevent the fluid for cooling from leaking at a connecting portion between the cooling roller and a circulating pump for a long time period, and a structure for sealing at the connecting portion becomes complicated.